Small, compact x-ray tubes have experienced widespread adoption in x-ray fluorescence (XRF) spectroscopy and x-ray diffraction (XRD) instruments, for a wide range of industrial and other applications. X-ray tubes conventionally emit radiation in a divergent manner. In the past, obtaining an illumination spot size of sufficient intensity on a small sample area typically necessitated increasing the power supplied to expensive, high-powered tubes or other sources. The ability to focus x-ray radiation to smaller spots with higher intensities, using focusing x-ray optics, has enabled reductions in the size and cost of x-ray tubes, and x-ray systems have therefore been proliferating beyond the laboratory to in-situ, field uses. U.S. Pat. Nos. 6,934,359 and 7,072,439, incorporated by reference herein in their entirety and assigned to X-Ray Optical Systems, Inc., the assignee of the present invention, disclose monochromatic wavelength dispersive x-ray fluorescence (MWD XRF) techniques and systems, using doubly curved crystal optics in the excitation and/or detection paths. The x-ray optic-enabled systems described in these patents have enjoyed widespread success beyond the laboratory, for measuring sulfur in petroleum fuels in a variety of refinery, terminal, and pipeline environments.
In such systems, precise optic alignment along an axis defined by a source and sample spot may be required, as illustrated in commonly-assigned U.S. Pat. No. 7,035,374, incorporated by reference in its entirety, which proposes an arrangement of curved, monochromating optics around a central axis operating according to Bragg diffraction conditions. FIG. 1a is a representative isometric view of this x-ray optic arrangement 150 having a curved optic 152, an x-ray source location 154, and an x-ray target location 156. X-ray source location 154 and x-ray target location 156 define a source-to-target transmission axis 162. Optic 152 may include a plurality of individual optic crystals 164, all of which are generally required to be arranged symmetrically about axis 162.
FIG. 1b is a cross-sectional view taken along section lines 1b-1b of FIG. 1a, wherein the surface of optic 152, x-ray source location 154, and x-ray target location 156 define one or more Rowland (or focal) circles 160 and 161 of radius R for optic 152. Those skilled in the art will recognize that the number and orientation of the Rowland circles associated with crystal optic 152, or individual crystals 164, will vary with the position of the surface of optic crystal 152, for example, the variation of the toroidal position on optic crystal 152.
The internal atomic diffraction planes of optic crystal 152 also may not be parallel to its surface. For example, as shown in FIG. 1b, the atomic diffraction planes of crystal 152 make an angle γ1 with the surface upon which x-rays are directed, at the point of tangency 158 of the surface and its corresponding optic circle 160 or 161. θB is the Bragg angle for crystal optic 152 which determines its diffractive effect. Each individual optic crystal can in one example be fabricated according to the method disclosed in commonly assigned U.S. Pat. No. 6,285,506, entitled “Curved Optical Device and Method of Fabrication,” incorporated herein by reference in its entirety.
All individual crystals 164 should be aligned to the source-to-target axis 162, for proper Bragg conditions. Improvement in optic alignment, especially for such multiple-crystal optics, therefore remains an important area of interest.
Another issue, which particularly affects volume manufacturing, is the need to align disparate components which may be purchased from different vendors. For example, the x-ray tubes, when purchased in quantities from a vendor, may have source x-ray spots which are not consistently centered relative to their own housings. Re-centering these x-ray tube spots is necessary, as an initial step in the alignment process for an entire x-ray source assembly.
Various optic/source combinations have already been proposed to handle thermal stability, beam stability, and alignment issues, such as those disclosed in commonly assigned U.S. Pat. Nos. 7,110,506; 7,209,545; and 7,257,193. Each of these patents is also incorporated herein by reference in its entirety. In particular, U.S. Pat. No. 7,209,545 (entitled “X-Ray Source Assembly Having Enhanced Output Stability, and Fluid Stream Analysis Applications Thereof”) and U.S. Pat. No. 7,257,193 (entitled “X-Ray Source Assembly Having Enhanced Output Stability Using Tube Power Adjustments and Remote Calibration”) address certain tube/optic alignment problems during source operation with real-time, corrective feedback approaches for alignment between the tube focal spot, optic, and output focal spot. Sensors are used to detect various operating conditions, and mechanical and/or thermal adjustments are made to correct for instabilities, including misalignments. These types of systems are necessary and valuable for certain applications, but can also increase the cost and complexity of fielded systems.
Tube-optic-target alignment therefore remains an important design criteria, especially for small source, lower cost, volume-manufactured “in the field” systems requiring precise alignment, ideally accomplished at the factory with no further need to re-align in the field.